Handle Errors

public class LibraryException extends Exception {}

Exactly Module 13's custom exception pattern — a one-line class that lets library-specific failures be caught and handled distinctly from generic Apex exceptions.

public Checkout__c checkOutBook(Id bookId, String memberName) {
    Book__c book = [SELECT Id, Is_Checked_Out__c FROM Book__c WHERE Id = :bookId];

    if (book.Is_Checked_Out__c) {
        throw new LibraryException('This book is already checked out.');
    }

    book.Is_Checked_Out__c = true;
    update book;

    Checkout__c checkout = new Checkout__c(Book__c = bookId, Member_Name__c = memberName, Checkout_Date__c = Date.today());
    insert checkout;
    return checkout;
}

Checking Is_Checked_Out__c before doing anything else prevents exactly the "two members borrow the same physical book" scenario Lesson 4 flagged as the reason Book__c and Checkout__c must stay in sync.

public void returnBook(Id checkoutId) {
    Checkout__c checkout = [SELECT Id, Book__c, Return_Date__c FROM Checkout__c WHERE Id = :checkoutId];

    if (checkout.Return_Date__c != null) {
        throw new LibraryException('This checkout has already been returned.');
    }

    checkout.Return_Date__c = Date.today();
    update checkout;

    Book__c book = [SELECT Id FROM Book__c WHERE Id = :checkout.Book__c];
    book.Is_Checked_Out__c = false;
    update book;
}

A Checkout__c with a Return_Date__c already set means that borrowing event is closed — trying to return it again is a genuine misuse, worth a clear LibraryException rather than silently double-processing the return.